1. Field of the Invention
The present invention relates generally to a hub clutch suitable for use in part-time four-wheel drive vehicles (referred to as part-time 4WD vehicles, hereinafter), and more specifically to a hub clutch low in noise, low in 2WD fuel consumption rate, long in life, and stable in function by protecting the hub clutch from external foreign matters.
2. Description of the Prior Art
In a part-time four wheel drive vehicle, for instance, when the vehicle travels in two wheel drive (referred to as 2WD) mode, the power transmission system including a driven-side propeller shaft, a differential gear, a vehicle wheel shaft, etc. are driven by a rotational power supplied from driven-side vehicle wheels. In order to reduce noise, lower fuel consumption rate, and prolong power transmission parts life, etc., a hub clutch is usually used to decouple the driven side vehicle wheels from the power transmission system.
An example thereof is disclosed in Japanese Unexamined Published (Kokai) Patent Application No. 61-238516 as shown in FIG. 1, in which the hub clutch actuating (coupling and/or decoupling) means is operated by pressure.
In this prior-art hub clutch, a drive gear 201 fixed to a wheel shaft 203 is engaged (coupled) with or disengaged (decoupled) from a slide gear 207 slidably moved relative to a wheel hub side housing 205 via a diaphragm 215 moved by a vacuum or an atmospheric pressure.
In more detail, the housing 205 is rotatably supported by a spindle 213 via a pair of bearings 209 and 211. The flexible diaphragm 215 separates the housing 205 into two right and left side chambers. A plate 217 movable together with the diaphragm 215 in the right and left (axial) directions engages or disengages the slide gear 207 with or from the drive gear 201. An airtight chamber 219 formed on the left side of the diaphragm 215 in FIG. 1 is connected to an engine intake manifold via a passage 221 which is switched to vacuum pressure or atmospheric pressure through a control valve 223 disposed midway of the passage 221. When the airtight chamber 219 is kept under vacuum condition, the diaphragm 215 is moved in the leftward direction, so that the plate 217 moves the slide gear 207 in the leftward direction against the biasing force of a coil spring 225 to engage the slide gear 207 with the drive gear 201; that is, the housing 205 is coupled with the wheel shaft 203 to realize a 4WD mode.
On the other hand, when the airtight chamber 219 is kept under atmospheric pressure, since the slide gear 207 is moved by the biasing force of the coil spring 225 in the rightward direction to disengage the slide gear 207 from the drive gear 201; that is, the housing 205 is decoupled from the wheel shaft 203 to realize a 2WD mode to reduce noise and economize power without rotating a propeller shaft.
Further, in FIG. 1, air ports 229 are formed in a right side partition chamber 227 (on the right side from the diaphragm 215) to keep the partition chamber 227 under atmospheric pressure.
In the above-mentioned prior-art hub clutch, however, when the wheel shaft 203 is coupled to the housing 205 during 4WD travel, since a high vacuum is always introduced into the airtight chamber 219, external contaminants such as dust and muddy water tend to enter the chamber 219 through each seal portion. Therefore, there exists a problem in that such contaminants move to the coupled portion (e.g. slide gear 201) or the sliding portion (e.g. bearings 209 and 211). Further, the movement of the diaphragm 215 is prevented by ice which has formed from water that has entered through the air ports 229 into the partition chamber 227 or by solidified mud. As described above, there exists a problem in that the clutch function and durability are deteriorated markedly.
Further, when the vacuum fluctuates during the 4WD travel and therefore the diaphragm actuation force is reduced below the biasing force of the coil spring 225, the slide gear 207 and the drive gear 201 are disengaged by the force of the coil spring 225, so that there exists another problem in that the hub clutch is returned to the 2WD mode.